The present invention relates to an atmospheric gas generator for heat-treating a metal material, and in particular to an apparatus for generating a carrier gas or an atmospheric gas for performing heat treatment to carburize, refine or carbonitrify a metal material.
An atmospheric gas for heat treatment performed to carburize, refine or carbonitrify a metal material is generally generated either by a metamorphic furnace system by which hydrocarbon gas in a metamorphic furnace is reacted with air by a catalyst, or by a dropping system by which a liquid organic solvent (typically, alcohol such as methanol; hereinafter, referred to simply as "alcohol") is thermally decomposed.
With respect to the metamorphic furnace system, it is difficult to generate an atmospheric gas having a stable composition without an experienced artisan in operation and maintenance of the metamorphic furnace. Thus, the dropping system, which does not involve such complicated operations or maintenance, has become a target of attention as a system for supplying an atmospheric gas having a stable composition in a heat treatment furnace.
There are two types of dropping systems. According to one type, an atmospheric gas is generated by thermally decomposing, in the furnace, only an alcohol which includes an additive for fixing the carbon concentration of the post-decomposition atmospheric gas. According to the other type, an atmospheric gas is generated by putting an alcohol containing no additive and nitrogen gas into the furnace. The latter type has been used recently mainly due to its flexibility in adjusting the carbon concentration in the atmospheric gas and its effect of reducing an oxide layer on a surface of a metal product to be heat-treated.
It is important that the atmospheric gas used for heat-treating a metal material should have a stable composition. Accordingly, when a dropping system is used, it is necessary to control the gas supplied to the furnace to have a stable flow rate and to provide a heat source which is sufficient for the alcohol to be decomposed.
Conventionally known methods for supplying alcohol and nitrogen gas into a heat treatment furnace include, for example: (1) thermally decomposing alcohol in a heating chamber provided inside or outside the furnace and supplying the generated gas into the furnace, (2) supersonically atomizing alcohol in the furnace, (3) supplying a mixture of gas and alcohol vapor, and (4) supplying alcohol and nitrogen gas through a nozzle to the furnace.
Method (1) is disclosed in Japanese Laid-Open Publication No. 62-60818, Japanese Publication for Opposition No. 63-50430, and Japanese Publication for Opposition No. 63-47771.
For example, a method for supplying an atmospheric gas disclosed in Japanese Laid-Open Publication No. 62-60818 is carried out as shown in FIG. 6. Liquid alcohol is dropped to a decomposing chamber 62 in a heat treatment furnace 61 through an alcohol introduction pipe 60 and heated by the temperature inside the heat treatment furnace 61 and a heater 63 to be decomposed into gas. The resultant gas is taken out of the decomposing chamber 62 to a position slightly above a stirring rotating fan 64 through an outlet 65. Then, the gas is stirred by the stirring rotating fan 64 to be diffused in the heat treatment furnace 61 outside the decomposing chamber 62.
Method (1) is alternatively carried out as shown in FIG. 7. Liquid alcohol supplied from the alcohol introduction pipe 60 through a vaporizing device 66 is heated by a heater 67 in the vaporizing device 66 to be in a vaporized state. The vaporized alcohol is guided to the heat treatment furnace 61 through an outlet 68 of the alcohol introduction pipe 60, the outlet 68 being located in the vicinity of the stirring rotating fan 64. Then, the vaporized alcohol is stirred by the stirring rotating fan 64 to be diffused in the heat treatment furnace 61.
Japanese Publication for Opposition No. 63-50430 discloses a method of providing a double chamber furnace including an outer chamber and an inner chamber, so that gas obtained by decomposing alcohol in the outer chamber is mixed with nitrogen gas and the resultant mixture is supplied to the inner chamber as an atmospheric gas.
However, these methods require a furnace having a more complicated structure and also require heavy maintenance. Moreover, the structure of the heat treatment furnace would need to be altered when switching between use of a dropping system and metamorphic furnace system for supplying the atmospheric gas.
Method (2) is disclosed in Japanese Publication for Opposition No. 63-47772. According to this method, unless the alcohol is in an ideal position in the furnace, the alcohol vapor accumulates on the bottom of the furnace due to the high specific gravity thereof. Then, the alcohol vapor directly contacts the metal product to be heat-treated, thus imposing an adverse effect on the quality of the product. As shown in the figure in the Japanese Publication for Opposition No. 63-47772, this method requires a supersonic forcible atomizing device, and therefore requires a large power source and ongoing maintenance.
Method (3) is disclosed in Japanese Laid-Open Publication No. 59-53676. According to this method, an atmospheric gas is generated by vaporizing alcohol by a vaporizer outside a heat treatment furnace and decomposing the alcohol in a hearth filled with fire-resistant particles. This method requires a furnace having a special structure including a vaporizer and a furnace floor. Existing furnaces cannot be used to generate an atmospheric gas from alcohol. This method further requires a heat source for the vaporizer, and maintenance of the vaporizer and the hearth.
Method (4) is disclosed in Japanese Publication for Opposition No. 63-47771 and is carried out as shown in FIG. 8. Liquid alcohol introduced into the heat treatment furnace 61 through an alcohol introduction pipe 71 of a nozzle 70 is mixed with nitrogen gas in the heat treatment furnace 61. The nitrogen gas is, while rotating by spiral grooves 72a at an end of a nitrogen introduction pipe 72, being expelling forcefully in the direction in which the alcohol introduction pipe 71 is inserted into the heat treatment furnace 61. The spiral grooves 72a are formed on an inner wall of the nitrogen introduction pipe 72. The liquid alcohol flows along the wall 61a of the heat treatment furnace 61 while being stirred by the stirring rotating fan 64 together with additional gas which is supplied through an additional gas introduction pipe 73.
According to this method, unless the nozzle 70 for supplying the alcohol via the alcohol introduction pipe 71 and the nitrogen gas via the nitrogen introduction pipe 72 is provided in the vicinity of the stirring rotating fan 64, the gas and alcohol expelled forcefully from the nozzle 70 is directly contacts the metal product to be heat-treated, thereby imposing an adverse effect on the quality of the product. This problem restricts the configuration of the heat treatment furnace 61. Moreover, when the supply of the atmospheric gas is stopped, the liquid alcohol remaining in the alcohol introduction pipe 71 is thermally decomposed and clogs the nozzle 70 as soot.